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Originally Posted by CoombellKid
I've looked through a few myself, and compared even a C9.25 with my own
8" newt. The C9.25 show no more than my 8" newt, infact I was un-impressed
as at the time my 8" was clearly showing detail you were seeing in the C9.25.
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One has to be careful of such comparisons, the seeing may have been limited to 8" cells on that night. However its generally accepted that a simple rule of thumb, assuming nice smooth optical surfaces is that you subtract the diameter of the central obstruction from the aperture to give you equivelent performing scopes in terms of equal contrast transfer function ( or MTF )
In other words , a C9.25 , 8inch Newt and 6" Apo refractor may all be capable of giving similar views , but I would say the 8" Newt would have the best potential for smooth wavefront with inly two surfaces in the optical train compared to 4 in the SCT.
Mark